Mutations occurring spontaneously or induced by exogenous genotoxicants are a root cause of cancer. Nearly all genotoxicant-induced mutations result from DNA damage and replication of the damaged DNA by specialized translesion synthesis (TLS) DNA polymerases that are less accurate than normal replicative DNA polymerases. TLS polymerases are also highly error-prone when copying undamaged DNA, thus constituting a persistent source of genomic instability that needs to be controlled to avoid disease. While the mechanisms of TLS polymerase recruitment to the sites of DNA damage are understood relatively well, the mechanisms that restrict their participation in the normal replication are much less clear. The PI's laboratory has discovered that the participation of DNA polymerase ? (Pol?) in the copying of undamaged DNA is promoted by a variety factors that impede the progression of the replication, including defects in the normal replication machinery, fork stalling at natural impediments and treatment with therapeutic replication inhibitors. This proposal seeks to define the global mechanisms that regulate the extent of error-prone synthesis by Pol? in vivo in DNA damaging and physiological conditions. In Specific Aim 1, we will determine the role of checkpoint dependent elevation of dNTP pools in shaping the error signature of Pol?. In Specific Aim 2, we will determine how the contribution of Pol? to DNA synthesis is regulated by the replication dynamics and fork asymmetry. The yeast Saccharomyces cerevisiae model system will be utilized in the studies proposed in Aims 1 and 2, with the goal of using the data obtained in yeast to further advance our understanding of the mechanisms of mutagenesis in human cells. In Specific Aim 3, we will characterize the mechanism of the mutagenic response to replication defects in human cells. The proposed work will lead to a better understanding of the mutagenic processes that operate in normal cells, as well as those induced by environmental genotoxicants or therapeutic interventions. In the long run, learning to manipulate these processes will help reduce cancer incidence, delay progression and improve therapy outcome.